Identification of Urban Effect on a Single Extreme High Temperature Event

On basis of selecting reference stations by using a relatively objective standard, this paper analyzes the influence of urban heat island (UHI) intensity (I_UHI) on spatial and temporal temperature characteristics in a high temperature process of Beijing from 2 July to 6 July 2010, by applying hourly data of high density automatic weather stations and NCEP reanalysis data. The maximum temperature (T_max) during the process is more than 35.0 ℃ for each day, and the five-day mean T_max breaks the records at Beijing Weather Station since 1951. Continental warm high pressure controls most parts of China in the high-temperature process, and Beijing is in front of the high-pressure ridge. Major causes for the high-temperature process in Beijing are strong subsidence and the fohn effect of the northwesterly flowing over the Taihang Mountains. Affected by UHI effect, the distribution of the T_max, minimum temperature (T_min), and mean temperature (T_mean) exhibits a high-temperature area in the center of the city, with the mean I_UHI gradually decreasing from the within fourth-ring area to the fifth-sixth-ring areas. The I_UHI of T_min is obviously larger, reaching 2.93 ℃, 1.87 ℃, and 1.43 ℃, respectively, for the central urban area, fourth-fifth-ring area, and fifth-sixth-ring area. The I_UHI of T_max is smaller, although the temperatures are as high as 1.45 ℃, 0.96 ℃, and 0.72 ℃, respectively, for the three urban zones mentioned above. The night I_UHI high values within the fourth ring during 3-6 July are more than 3.00 ℃, and the maximum I_UHI reaches 5.50 ℃ in the early morning on 6 July. The daytime I_UHI is relatively small, and the I_UHI shows a negative value in the morning of 2 July likely because the heating rate at reference stations is greater than that in the urban areas. The I_UHI diurnal variation of the three urban zones is nearly synchronic, and is characterized by a steadily strong I_UHI stage from 2100 LST to 0500 LST and a steadily weak I_UHI stage from 0800 LST to 1800 LST, with the periods of 0500-0800 LST and 1800-2100 LST undergoing a rapid decline and increase of the I_UHI, respectively. Therefore, the UHI effect has a marked influence on the strength and spatial distribution of the single extreme high temperature event in the urban area of Beijing.